This invention relates to continuous casting of steel strip in a strip caster, particularly a twin roll caster.
In a twin roll caster molten metal is introduced between a pair of contra-rotated horizontal casting rolls which are cooled so that metal shells solidify on the moving roll surfaces and are brought together at the nip between them to produce a solidified strip product delivered downwardly from the nip between the rolls. The term xe2x80x9cnipxe2x80x9d is used herein to refer to the general region at which the rolls are closest together. The molten metal may be poured from a ladle into a smaller vessel from which it flows through a metal delivery nozzle located above the nip so as to direct it into the nip between the rolls, so forming a casting pool of molten metal supported on the casting surfaces of the rolls immediately above the nip and extending along the length of the nip. This casting pool is usually confined between side plates or dams held in sliding engagement with end surfaces of the rolls so as to dam the two ends of the casting pool against outflow, although alternative means such as electromagnetic barriers have also been proposed.
When casting steel strip in a twin roll caster, the strip leaves the nip at very high temperatures of the order of 1400xc2x0 C. and it suffers very rapid scaling due to oxidation at such high temperatures. Such scaling results in a significant loss of steel product. For example, 3% of a 1.55 mm thick strip (typical scale thickness microns) can be lost from oxidation as the strip cools. Moreover, it results in the need to descale the strip prior to further processing to avoid surface quality problems such as rolled-in scale and this causes significant extra complexity and cost. For example, the hot strip material may be passed directly to a rolling mill in line with the strip caster and thence to a run out table on which it emerging from the strip caster progresses so rapidly that it becomes necessary to install descaling equipment to descale the material immediately before it enters the in line rolling ill. Even in cases when the strip is cooled to coiling temperature without hot rolling, it will generally be necessary to descale the strip either before it is coiled or in a later processing step.
To deal with the problem of rapid scaling of strip emerging from a twin roll strip caster it has been proposed to enclose the newly formed strip within a sealed enclosure, or a succession of such enclosures, in which a controlled atmosphere is maintained in order to inhibit oxidation of the strip. The controlled atmosphere can be produced by charging the sealed enclosure or successive enclosures with non-oxidizing gases. Such gases can be inert gases such as nitrogen or argon or exhaust gases from fuel burners.
U.S. Pat. No. 5,762,126 discloses an alternative relatively cheap and energy efficient way of limiting exposure of the high temperature strip to oxygen. The strip is caused to pass through an enclosed space from which it extracts oxygen by the formation of scale and which is sealed so as to control the ingress of oxygen containing atmosphere whereby to control the extent of scale formation. In this method of operation, it is possible to rapidly reach a steady state condition in which scale formation is brought to low levels without the need to deliver a non-oxidizing or reducing gas into the enclosure.
U.S. Pat. No. 5,816,311 discloses a way of controlling the extent of scale formation by providing downstream a chamber where nozzle groups spray a quenching medium onto the strip. The quenching medium was a methyl alcohol, water, or mixture of methyl alcohol and another quenching medium which is liquid at room temperature. It was expected that water spraying in a nitrogen atmosphere would lead to unacceptable levels of oxidation as water contains dissolved oxygen and the breakdown of water (steam) to oxygen and hydrogen would provide further oxidation; however, it was surprisingly and unexpectedly found as described in the ""311 patent that it was possible to limit the thickness of oxide on the strip to no more than 0.5 microns. Additionally, it was surprisingly found that these levels of oxide were tolerable for cold rolling without pickling and then metal coating of the strip. This quenching of the steel strip was found, however, to result in uneven cooling of the steel strip introducing stresses and other defects in the strip.
We have now determined that a substantially non-oxidizing atmosphere can be cheaply and effectively produced within an enclosure for the hot steel strip by introducing water in a fine mist spray to generate steam within the enclosure. The steam generation greatly increases the gaseous volume within the enclosure so as to produce a superatmospheric pressure which substantially prevents the ingress of atmospheric air. It can also produce an increased level of hydrogen gas within the enclosure to significantly reduce the oxygen level in the enclosure and retard the rate of oxidation of the strip. Since the casting rolls cannot be exposed to water or steam without risking catastrophic disturbance of the casting pool, it is necessary to isolate the enclosure in which steam is generated from the cooling rolls. However, it has been found that introducing water in a fine mist spray to generate steam within the enclosure produced increased levels of hydrogen gas while tending to avoid liquid water contact with the steel strip resulting in uneven cooling of the strip.
According to the present invention there is provided a method of continuously casting steel strip comprising:
supporting a casting pool of molten steel on one or more chilled casting surfaces;
moving the chilled casting surface or surfaces to produce a solidified steel strip moving away from the casting pool;
guiding the solidified strip successively through first and second enclosures as it moves away from the casting pool;
sealing the first and second enclosures to restrict ingress of atmospheric air; and
introducing water into the second enclosure in the form of fine mist to generate steam within the second enclosure and thereby to produce a superatmospheric pressure in that enclosure substantially excluding ingress of atmospheric air.
The first enclosure should be of sufficient length to minimize the possibility of migration of water vapor into the region immediately below the casting rolls. This is accomplished by the first enclosure, and then the second enclosure, being provided to surround the solidified steel strip as the strip moves away from the casting pool such that the strip can be exposed to separate atmospheric conditions in the first enclosure, and thereafter the second enclosure. The strip may exit the first enclosure at a temperature in the range 1300xc2x0 C. to 1150xc2x0 C., preferably about 1220xc2x0 C.
The water may be introduced through one or more fine mist sprays directed onto a face of the steel strip as it passes through the second enclosure. More specifically, the water is preferably introduced through one or more mist sprays directed downwardly onto the upper face of the steel strip. Because the water is introduced in a fine mist spray, the water is converted into steam in the second enclosure while tending to avoid liquid water contact with the steel strip.
In order to produce the spray mist, water may be forcibly propelled by a gas propellant through one or more mist spray nozzles. The gas propellant may be an inert gas, for example nitrogen. The introduction of the water spray mist into the second enclosure produces an increased level of hydrogen gas therein, while tending to avoid liquid water contact with the steel strip and uneven cooling of steel strip.
The strip may be passed from the first enclosure to the second enclosure through a pair of pinch rolls. In that case the pinch rolls may be operated to reduce the strip thickness by up to 5%, and preferably of the order of 2%.
The first and second enclosures may initially be purged with an inert gas, for example nitrogen, before commencement of casting of said strip so as to reduce the initial oxygen content within the enclosures. Such purging may for example reduce the initial content within the enclosures to between 5% to 10% and usually to even lower levels such as 3%.
During casting of said strip the first enclosure may be continuously charged with an inert gas, for example nitrogen. Alternatively, the oxygen content in the first enclosure may be maintained at a level less than the surrounding atmosphere by continuous oxidation of the strip passing therethrough in the manner disclosed in U.S. Pat. No. 5,762,126.
The invention further provides apparatus for casting steel strip comprising:
a pair of generally horizontal casting rolls forming a nip between them;
metal delivery system to deliver molten steel into the nip between the casting rolls to form a casting pool of molten steel supported on the rolls;
a cooling system to chill the casting rolls;
a drive system to rotate the casting rolls in mutually opposite directions whereby to produce a cast strip delivered downwardly from the nip;
strip guides to guide the strip delivered downwardly from the nip through a transit path which takes it away from the nip;
a first enclosure to confine the strip throughout said transit path which enclosure is sealed to control ingress of atmospheric air;
a second enclosure to receive the strip after it has passed through the first enclosure which second enclosure is also sealed to control ingress of atmospheric air; and
one or more water sprays operable to spray water into the second enclosure in the form of a fine mist so as to generate steam within the second enclosure.
Preferably, the one or more water sprays comprise one or more water mist spray nozzles mounted within the second enclosure and operable to spray water mist onto the upper face of steel strip. The water mist is such that water generates steam in the second enclosure and tends to avoid liquid water from contacting the steel strip.
In the illustrative method according to the invention, the solidified steel strip may be delivered to a hot rolling mill in which it is hot rolled as it is produced. The strip may exit the second enclosure before entering the rolling mill and in this case the enclosure may comprise a pair of pinch rolls between which the strip passes to exit the second enclosure. However, it is preferred that the strip remain within the second enclosure at its entry into the rolling mill. This may be achieved by sealing the second enclosure against rolls or a housing of the rolling mill.